Metal matrix composite components, such as Aluminum/Silicon Carbide (Al/SiC) electronic packages, are typically plated to prevent corrosion and to allow devices to be brazed or soldered on them. For instance, composite electronic packages are normally plated with gold and then feed-throughs such as glass-to-metal feed-throughs are brazed or soldered into the package. Electric components are then bonded or soldered into the package and wire-bonded together. The package is then sealed.
Al/SiC electronic packages have been used for airborne or space borne radar systems, such as phased array tracking systems. The composite packages hold the transmitter and receiver devices which are typically made of gallium arsenide and alumina. The plating of these packages is critical to the bonding, corrosion resistance, hermeticity, and long-term reliability of the system.
An Al/SiC composite, as an example, consists of silicon carbide particles in an aluminum matrix. Different amounts of SiC particles may exist at the surface of the part, depending on the composite system and the fabrication method. A problem exists in that the SiC particles do not plate with conventional aluminum plating processes.
In an attempt to overcome the problem, many different methods have been investigated to plate Al/SiC packages. These methods have included sputtering of aluminum or other metals to create a continuous metal surface.
Standard aluminum plating processes include a zincate coating followed by an electroless coating of nickel, additional nickel, and finally a layer of gold. However, if this process is used on Al/SiC, the SiC particles will be left unplated. In some cases where the SiC particles are below 10 microns in size and comprise 50% or less of the surface area, the plating may bridge the particles and create a smooth plated surface with unbonded voids underneath. This plating, is prone to blistering at elevated temperatures above 300.degree. C. The gap between the bridged metal and the SiC particles may trap chemicals which later react with the SiC and aluminum which causes gas evolution and blisters. Nickel phosphorous plating may leave phosphoric acid on the surface of the particles which can cause the particles to dissolve at temperatures above 200.degree. C.
Another problem with plating composites is that the composite parts plated with standard plating processes are unable to withstand many brazing processes. Components are exposed to high temperature during the brazing and die attachment production phases. Al/SiC components are typically assembled with gold/tin or gold/germanium brazes. These brazes typically require a temperature exposure of 300.degree. to 400.degree. C., respectively. Al/SiC packages plated with standard aluminum plating processes tend to blister above 350.degree. C. Composite components which are brazed typically fail to maintain hermeticity and corrode upon long term exposure. These failures are due to interconnecting porosity found around the SiC particles. Corrosion and blistering cause failure to eventually occur even if the packages were hermetic when assembled.